This invention is directed to processes for the conversion of material boiling in the Vacuum Gas Oil boiling range to high quality middle distillates and/or naphtha and lighter products, and more particularly to a multiple stage process using a single hydrogen loop.
In the refining of crude oil, gas oil hydrocrackers are used to convert heavy gas oils to lighter products using a single reaction stage or multiple reaction stages. In most instances, the various reaction stages operate at similar pressure levels. Where pressure levels are different, separate hydrogen loops are employed. Multiple reaction stages are used to achieve the following:
high conversion employing minimum reactor volume and catalyst volume overall
better product qualities
lower hydrogen consumption
The use of multiple reaction systems however involves more equipment including multiple expensive high-pressure pumps and compressors.
U.S. Pat. No. 5,980,729 discloses a configuration with multiple reaction zones in a single hydrogen loop. The process uses a hot stripper downstream of the denitrification/desulfurization zone. Liquid from the hot stripper is pumped to the hydrocracking reactor upstream of the hydrotreating reactor. Recycle oil from the fractionation section is also pumped back to the hydrocracking reactor.
The process configuration for the instant invention is different from U.S. Pat. No. 5,980,729 in many aspects. The primary reactor is a combination hydrotreating-hydrocracking reactor that uses no recycle liquid.
Liquid from the hot stripper downstream of the reactor is reduced in pressure to a subsequent reaction stage where hydrocracking reactions are completed. No pump is involved in the transfer of liquid. Also, the second hydrocracking stage operates at lower pressure than the primary reaction stage.
With this invention, moderate to high conversion can be achieved using a single hydrogen loop. Product quality can be modulated to just meet specifications, eliminating product loss and saving hydrogen. The pressures of the reaction stages are maintained at levels suited for particular types of feed characteristics, i.e., only the first stage reactor that processes the most difficult feed must operate at the highest pressure level. High-temperature, high-pressure pumps are not involved in the process. The second hydrocracking reactor stage can operate in either co-current or counter-current mode with respect to the reaction gas, which in the present invention is primarily make-up hydrogen. The second hydrocracking reaction stage is fed with high purity make-up hydrogen to maximize hydrogen partial pressure. The second stage is loaded with very high activity catalyst that can be used for hydrocracking at relatively low pressures. The invention is summarized below:
1. A method for hydroprocessing a hydrocarbon feedstock, said method employing multiple reaction zones within a single reaction loop, comprising the following steps:
(a) passing a hydrocarbonaceous feedstock to a first hydroprocessing zone having one or more beds containing hydroprocessing catalyst, the hydroprocessing zone being maintained at hydroprocessing conditions, wherein the feedstock is contacted with catalyst and hydrogen;
(b) passing the effluent of step (a) directly to a hot high pressure separator, wherein the effluent is contacted with a hot, hydrogen-rich stripping gas to produce a vapor stream comprising hydrogen, hydrocarbonaceous compounds boiling at a temperature below the boiling range of the hydrocarbonaceous feedstock, hydrogen sulfide, ammonia, and a bottoms stream comprising hydrocarbonaceous compounds boiling in approximately the same range of said hydrocarbonaceous feedstock along with a portion of the hydrocarbonaceous compounds boiling in the diesel boiling range;
(c) passing the vapor stream from step (b) after cooling and partial condensation to a hot hydrogen stripper containing at least one bed of hydrotreating catalyst, where it is contacted countercurrently with hydrogen, while the liquid stream of step (b) is passed to a second stage reactor;
(d) passing the overhead vapor stream from the hot hydrogen stripper of step (c), after cooling and contacting with water, to a first cold high pressure separator where hydrogen, hydrogen sulfide and light hydrocarbonaceous gases are removed overhead and a liquid stream comprising naphtha and middle distillates is passed to fractionation, thereby removing most of the ammonia and some of the hydrogen sulfide (as ammonium bi-sulfide in the sour water stream) as it leaves the cold high-pressure separator;
(e) passing the liquid stream from the hot hydrogen stripper of step (c) to a bed of hydroprocessing catalyst in the second reactor stage wherein the liquid is contacted under hydroprocessing conditions with the catalyst, in the presence of hydrogen;
(f) passing the overhead from the cold high pressure separator of step (d) to an amine absorber, where hydrogen sulfide is removed before hydrogen is compressed and recycled to hydroprocessing vessels within the loop;
(g) passing the hot high pressure separator bottoms of step (b) to a second reaction stage where it is contacted with at least one bed of hydrocracking catalyst in the presence of high purity hydrogen to produce a vapor stream and liquid effluent;
(h) passing the vapor stream of step (g) after cooling to a second cold high-pressure separator where a vapor stream is removed comprising primarily hydrogen and light hydrocarbonaceous gases;
(i) passing the liquid effluent of step (g), after cooling, to the cold high-pressure separator of step (h) to separate hydrogen and light hydrocarbonaceous gases from the liquid effluent;
(j) passing the vapor stream from steps (h) and (i) after further cooling and separation of condensate, to the make-up hydrogen compressor;
(k) passing the compressed hydrogen from the make-up hydrogen compressor to the primary reactor loop; and
(l) passing the liquid effluent from steps (h) and (i) to the fractionation system.